The present invention relates generally to light traps for absorbing light and, more particularly, to light traps including associated light focusing assemblies to direct light into the light trap.
In many optical systems light traps are used to absorb light. For example, light traps can be used to remove the directly transmitted light component in turbidity measurements, to perform specular component subtraction in color measurement, and to calibrate devices used for light related measurements, such as reflectometers. In this regard, calibration of a meter that measures reflection generally involves two steps. During the first step, an object having a highly, almost perfectly reflective surface is inserted in place of a test object. The meter then measures the reflection of light off of the object having the highly reflective surface to thereby establish an upper boundary calibration point to which subsequently measured test objects are compared. During the second step, a light trap having a low reflective surface is inserted in place of a test object, and the reflection of light off of the light trap establishes a lower boundary calibration point to which subsequently measured test objects are compared.
Thus, it is desirable for a light trap to absorb all or substantially all of the light that impinges the light trap. One conventional light trap 10, shown in FIG. 1A and described by French Patent Application No. 2,088,198, comprises an enclosure in the form of a body of revolution that includes a lateral wall 12 with an absorbent internal surface 12a. The light trap also includes an entry member 14 that is disposed longitudinally at one end of the enclosure. The entry member has an internal face 16, an external face 18 and a frustoconical opening 20 through the entry member and coaxial with the enclosure through which a light beam enters the enclosure. The frustoconical opening narrows in the direction from the internal face of the input member to the external face. The entry member includes an inclined surface on the external face around the frustoconical opening adapted to reflect luminous radiation encountering the inclined surface towards an absorbent peripheral surface facing the latter. To direct the light beams toward the absorbent internal surface of the lateral wall, the light trap also includes a conical body 22 attached to a back wall 24 of the light trap and coaxial with the enclosure, where the point of the cone faces the frustoconical opening.
Another conventional light trap is shown in FIG. 1B and described by U.S. Pat. No. 5,745,293. As shown, the light trap 26 of the ""293 patent comprises an enclosure that is in the form of a body of revolution and includes a lateral wall 28 with an absorbent internal surface 30. The light trap also includes an entry member 32 that is disposed longitudinally at one end of the enclosure, and includes an internal face 34, an external face 36 and a frustoconical opening 38 through the entry member and coaxial with the enclosure through which a light beam enters the enclosure. At the external face of the entry member, the frustoconical opening has a diameter equal to the diameter of the light beam to be eliminated. Also, the frustoconical opening narrows from the external face towards the internal face of the entry member. Further, a conical body 40 is attached to a back wall 42 of the light trap and is coaxial with the enclosure, where the point of the cone faces the frustoconical opening such that light entering through the opening strikes the cone and is directed toward the absorbent internal surface of the lateral wall.
Yet another conventional light trap is shown in FIG. 1C. As shown, the light trap 44 comprises an enclosure having a lateral wall 46 and defining an opening 48 through which a light beam enters the enclosure. Within the enclosure, the light trap includes a wedge-shaped member 50. The wedge-shaped member opens toward the opening and defines a forty-five degree interior angle. The wedge-shaped member includes two faces upon which two pieces of black glass 54 and 56 are disposed. Thus, the pieces of black glass extend from one another toward the opening at a forty-five degree angle.
Conventional light traps, such as those illustrated in FIGS. 1A, 1B and 1C are adequate for absorbing beams of light that enter the light traps parallel to the direction of the lateral walls of the respective light traps. But because of the design of such conventional light traps, conventional light traps will not absorb all of the beams of light that enter the respective light traps. In this regard, at least a portion of some beams of light, particularly those that enter the light traps at an angle close to perpendicular with the lateral walls, will reflect out of the light traps. The escape of light from light traps is disadvantageous. For example, in instances in which a light trap is utilized to calibrate a meter, the escape of light from a light trap skews the lower boundary calibration point.
In light of the foregoing background, embodiments of the present invention provide an improved light trap and light focusing assembly. In contrast to conventional light traps, the light trap and light focusing assembly of various embodiments of the present invention prevent most, if not all, reflections of light from escaping the light trap. According to one embodiment, the light trap includes a light focusing assembly and a light trap assembly. The light focusing assembly includes an access member and an intermediate member. The access member has a frustoconical shape and includes a first end and a second end having a larger cross-sectional area than the first end. The intermediate member also includes a first end and a second end. In this regard, the intermediate member defines an internal cavity therethrough, and the first end of the intermediate member is in optical communication with the second end of the access member. In another embodiment, the first end of the intermediate member has a larger cross-sectional area than the second end of the intermediate member. In this embodiment, the intermediate member has a frustoconical shape.
The light trap assembly includes a housing and a conical member. The housing defines an internal cavity that opens through one end thereof. In one embodiment, the housing includes a first end and a second end. In this embodiment, the first end of the housing defines the opening to the internal cavity. Also in this embodiment, the first end of the housing has a smaller cross-sectional area than the second end of said housing such that said intermediate member has a frusto-bell shape. In another embodiment, the housing has a cross-sectional area larger than a cross-sectional area of the intermediate member. The housing is positioned relative to the intermediate member proximate the second end of the intermediate member. For example, the housing can be positioned relative to the intermediate member such that the second end of the intermediate member extends into the internal cavity defined by the housing. The conical member, on the other hand, has a base and an opposing apex, and is disposed within the internal cavity defined by the housing. In this regard, the conical member can be disposed such that the apex of the conical member extends into the internal cavity defined by the intermediate member.
To facilitate the absorption of rays of light that enter the light trap, the access member, intermediate member and housing can each include an optically absorbent internal surface. Similarly, the conical member includes an optically absorbent external surface. For example, the internal surfaces of the access member, intermediate member and housing, as well as the external surface of the conical member, can each include an optically absorbent coating.
To absorb light, the access member defines an internal cavity that is capable of receiving at least one ray of light through the first end. Thereafter, the light trap is capable of facilitating specular propagation of the rays of light into the internal cavity of at least one of the access member, the intermediate member and the housing. During propagation of the rays of light, the light trap is capable of absorbing portions of the light to thereby limit propagation of the rays of light out of the internal cavity of the access member after the internal cavity has received the at least one ray of light. As the light trap facilitates specular propagation of the rays of light, the access member, intermediate member and housing can each include an internal surface having a predetermined specular gloss. Similarly, the conical member can include an external surface having the predetermined specular gloss.